The present invention is directed to ink compositions and imaging and printing processes thereof. More specifically, in one embodiment the present invention is directed to ink jet ink compositions comprised of a dendrimer core colored, including black, with a dye, or mixture of dyes covalently attached to the arms, or the arms, or branches, of the aforementioned dendrimer. One embodiment of the present invention is directed to an ink composition which comprises an aqueous liquid vehicle and a colored dendrimer, or dendricolorant, obtained by attaching a reactive dye or dyes to commercially available amino terminated, that is for example an amine group like NH.sub.2 is attached to the end of the arm farthest removed in distance from the core, which dendrimers can be of the first, second, third, or n-th generation, wherein is a number of less than 9, and more specifically is a number of from about 1 to about 8, and wherein the attachment reaction can be accomplished at room temperature in water. The resulting dispersion of colored dendrimers can be formulated into an ink by the addition, for example, of a cosolvent comprised of water and a glycol, like diethyleneglycol, thereby improving latency, which is the maximum time period, for example less than one hour, and from about 1 to about 10 minutes, over which an uncapped ink jet printhead can remain idle before noticeable deterioration of its jetting performances, and this addition can improve ink drying time, that is the time needed for an ink jet print to dry an extent such that it will not smear or offset upon handling or when placed in contact with another sheet of paper, which drying time can, for example, be less than one minute, or more specifically from about 10 to about 30 seconds. Also, the addition of glycol permits the adjustments of the ink viscosity from about 1.1 to about 4 centipoise, and preferably from about 1.1 to about 3.0 centipoise, and can permit adjustment of the ink surface tension. Viscosity and surface tensions are major contributing factors in the production of excellent quality prints on plain papers, that is prints with acceptable edge acuity, that is the sharpness of the image between the printed and nonprinted areas, minimal ink feathering on paper, high optical density, and characterized, for example, by no mottle in solid areas, or a desirable uniformity of solid area ink coverage. The effects of the ink surface tension and viscosity on the penetrability of the ink in a paper can be estimated by the Lucas-Washburn penetrability coefficient which is defined by (.gamma. cos .theta.)/.eta., where .gamma. and .eta. are the surface tension and the viscosity, respectively, of the ink which is absorbed on the paper and .theta. is the contact angle between the ink and the paper. Advantages of inks associated with the present invention in embodiments thereof include the preselection of the colors desired; excellent color intensity of the prints and images developed; control of the physical characteristics of the core by the selection of the dendrimer; the selection of the physical characteristics of the core including, for example its diameter, its hydrophobicity, and the chemical functionality of its surface allows the control of parameters, such as level of dye incorporation in the dendricolorant, the jettability of an ink comprised of these colorants; the generation of prints with excellent, usually the higher the optical density the darker the image, that is for example the image is substantially uniform in color, optical density, of from about 0.8 to about 1.5, depending on the color of the ink; excellent waterfastness of, for example, from about 80 to about 98 percent; resistance to highlighter marking; formation of clear images on transparencies; the use of a variety of plain papers, such as Xerox Corporation 4024.RTM. paper for ink jet printing; and the like. The inks of the present invention can be selected for a number of known ink jet printing methods and apparatus, including thermal ink jet, or bubble jet processes as described in, for example, U.S. Pat. No. 4,601,777, U.S. Pat. No. 4,251,824, U.S. Pat. No. 4,410,899, U.S. Pat. No. 4,412,224, and U.S. Pat. No. 4,532,530, the disclosures of which are totally incorporated herein by reference.
Ink jet printing systems can generally be classified by two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
Since drop-on-demand systems require no ink recovery, charging, or deflection, they are much simpler than the continuous stream type. There are two types of drop-on-demand ink jet systems. One type of drop-on-demand system has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. The relatively large size of the transducer prevents close spacing of the nozzles, and physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerances for drop velocity variation and directionality, thus impacting the system's ability to produce high quality copies. Drop-on-demand systems which use piezoelectric devices to expel the droplets also suffer the disadvantage of a slow printing speed.
The second type of drop-on-demand system is known as thermal ink jet, or bubble jet. With this type, there are apparently generated high velocity droplets and there is allowed very close spacing of the nozzles. The major components of this type of drop-on-demand system are an ink-filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle causing the ink in the immediate vicinity to evaporate almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands. When the hydrodynamic motion of the ink stops, the process is ready to start all over again. With the introduction of a droplet ejection system based upon thermally generated bubbles, commonly referred to as the "bubble jet" system, the drop-on-demand ink jet printers provide simpler, lower cost devices than their continuous stream counterparts, and yet have substantially the same high speed printing capability.
The operating sequence of the bubble jet system begins with a current pulse through the resistive layer in the ink filled channel, the resistive layer being in close proximity to the orifice or nozzle for that channel. Heat is transferred from the resistor to the ink. The ink becomes superheated far above its normal boiling point, and for water based ink, finally reaches the critical temperature for bubble formation or nucleation of around 280.degree. C. Once nucleated, the bubble or water vapor thermally isolates the ink from the heater and no further heat can be applied to the ink. This bubble expands until all the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor, which removes heat due to heat of vaporization. The expansion of the bubble forces a droplet of ink out of the nozzle, and once the excess heat is removed, the bubble collapses on the resistor. At this point, the resistor is no longer being heated because the current pulse has passed and, concurrently with the bubble collapse, the droplet is propelled at a high rate of speed in a direction toward a recording medium. The resistive layer encounters a severe cavitational force by the collapse of the bubble, which tends to erode it. Subsequently, the ink channel refills by capillary action. This entire bubble formation and collapse sequence occurs in about 10 microseconds. The channel can be refired after 100 to 500 microseconds minimum dwell time to enable the channel to be refilled and to enable the dynamic refilling factors to become somewhat dampened. Thermal ink jet processes are well known as indicated herein, and are described, for example, in U.S. Pat. Nos. 4,601,777; 4,251,824; 4,410,899; 4,412,224; and 4,532,530, the disclosures of each of which are totally incorporated herein by reference.
Known ink jet inks generally comprise a water soluble dye which is soluble in an ink vehicle such as water or a mixture comprising water and a water soluble or water miscible organic solvent. Inks comprising soluble dyes may exhibit many problems, such as poor waterfastness, poor lightfastness, clogging of the jetting channels as a result of solvent evaporation and changes in the solubility of the dye, dye crystallization, ink bleeding when prints are formed on plain papers, poor thermal stability, chemical instability, ease of oxidation, and low drop velocity. In addition, many of the dyes contained in inks may be potentially toxic or mutagenic. These problems can be minimized by replacing the dyes used in ink formulations with insoluble pigments. In general, pigments are superior to dyes with respect to waterfastness, lightfastness, image density, thermal stability, oxidative stability, the ability to perform intercolor ink mixing, compatibility with both coated/treated and plain papers, image edge acuity, reduced image feathering, and nontoxic and nonmutagenic properties.
Heterophase ink jet inks are known. For example, U.S. Pat. No. 4,705,567, the disclosure of which is totally incorporated herein by reference, discloses a heterophase ink jet ink composition which comprises water and a dye covalently attached to a component selected from the group consisting of poly(ethylene glycols) and poly(ethylene imines), which component is complexed with a heteropolyanion. In addition, U.S. Pat. No. 4,597,794 discloses an ink jet recording process which comprises forming droplets of an ink and recording on an image receiving material by using the droplets, wherein the ink is prepared by dispersing fine particles of a pigment into an aqueous dispersion medium containing a polymer having both a hydrophilic and a hydrophobic construction portion. The hydrophilic portion constitutes a polymer of monomers having mainly polymerizable vinyl groups into which hydrophilic portions such as carboxylic acid groups, sulfonic acid groups, sulfate groups, and the like are introduced. Pigment particle size may be from several microns to several hundred microns. The ink compositions disclosed may also include additives such as surfactants, salts, resins, and dyes.
U.S. Pat. No. 4,877,451 (Winnik et al.), the disclosure of which is totally incorporated herein by reference, discloses ink jet ink compositions comprising water, a solvent, and a plurality of colored particles comprising hydrophilic porous silica particles to the surfaces of which dyes are covalently bonded through silane coupling agents. In addition, copending application U.S. Ser. No. 07/369,003, the disclosure of which is totally incorporated herein by reference, there are illustrated ink jet inks and liquid developers containing colored particles comprising hydrophilic porous silica particles to the surfaces of which dyes are covalently bonded through silane coupling agents. The ink compositions of this patent are believed to be less stable in the printheads, and less stable during storage, disadvantages avoided, or minimized with the inks of the present invention.
In a patentability search report the following United States patents were recited: U.S. Pat. No. 4,705,567 relating, for example, to heterophase ink compositions comprised of water and a dye covalently attached to a polyethylene glycol, or polyethylene imine component, which component is complexed with a heteropolyanion; U.S. Pat. No. 4,623,689 which discloses, for example, an ink for ink jet recording wherein the ink contains a certain aqueous colored polymer, see the Abstract for example; and as collateral interest U.S. Pat. Nos. 4,664,708, 4,680,332, and 4,791,165. The disclosures of the aforementioned patents, and all other patents mentioned herein are totally incorporated herein by reference.
Copending application U.S. Ser. No. 544,564, the disclosure of which is totally incorporated herein by reference, relates, for example, to ink compositions which comprise an aqueous liquid vehicle and colored particles of an average diameter of 100 nanometers or less which comprise micelles of block copolymers of the formula ABA, wherein A represents a hydrophilic segment and B represents a hydrophobic segment, and wherein dye molecules are covalently attached to the micelles. In a specific embodiment of the copending application, the colored particles comprise micelles of block copolymers of the formula ABA having silica precipitated therein and dye molecules covalently attached to the micelles. Another embodiment of the copending application is directed to a printing process which comprises incorporating the ink thereof into an ink jet printing apparatus and causing droplets of the ink to be ejected in an imagewise pattern onto a substrate, thereby generating images on the substrate. Also, in another embodiment of the copending application there is disclosed an ink preparation process which comprises, in the order stated, (1) adding to water a block copolymer of the formula ABA, wherein A represents a hydrophilic segment and B represents a hydrophobic segment, thereby forming a dispersion of micelles of the block copolymer; (2) adding a water-soluble base to the dispersion, thereby bringing the pH of the dispersion to at least 8; (3) adding to the dispersion a solution comprising water and a reactive dye capable of reacting with the block copolymer, thereby forming colored polymeric micelles; and (4) admixing the colored micelles with an aqueous liquid vehicle to form an ink composition.
Heterophase inks containing pigment particles as colorants, however, also exhibit difficulties. For example, the particulate colorant may exhibit a tendency to settle out or separate from the liquid vehicle, particularly when the ink is stored for long periods of time. In addition, inks containing pigment particles as colorants tend to be opaque instead of transparent, which reduces their usefulness for printing images on transparencies for the purpose of overhead projection. Further, inks containing pigment particles as colorants tend to clog the narrow orifices of the printhead resulting in deterioration of the print quality. These and other disadvantages, such as poor resistance to rubbing, by another substrate or by hand are avoided, or minimized with the inks of the present invention.
While ink compositions are known a need remains for ink compositions exhibiting advantages of both dye-based inks and pigment-based inks. There is also a need for ink compositions with excellent waterfastness characteristics. A need also remains for ink compositions exhibiting acceptable lightfastness characteristics. Further, there is a need for ink compositions that are nontoxic and nonmutagenic. In addition, a need exists for ink compositions for which a wide variety of color choices exists. There is also a need for ink compositions that can be prepared by simple and economical processes. Further, there is a need for ink compositions suitable for printing on plain papers, coated or treated papers, and transparency materials. In addition, there is a need for ink compositions that when printed on substrates exhibit excellent optical density, low feathering, for example there is minimal undesirable bleeding of the ink in areas adjacent to the printed images, and excellent rub resistance, for example after strong hand rubbing for an extended period of time, about up to three minutes, the image is not removed or disturbed. There is also a need for ink compositions that when used to print on transparency materials generate images that project their original colors when light is passed through the image. A need also remains for ink compositions with acceptable thermal stability, for example the inks are not substantially adversely effected in the printhead by heat, and storage stability. Further, there is a need for ink compositions suitable for ink jet printing that do not induce clogging of the printhead. A need also remains for ink compositions that when printed on substrates exhibit no undesirable intercolor bleeding between areas of different color.